This invention relates generally to an apparatus for detecting the passage of multiple documents in a dispensing system, and, in particular, to an apparatus which monitors a currency dispenser in order to sense the presence of multiple or double bills.
With the acceptance of automated financial terminals increasing rapidly, the role of the currency dispenser is growing more significant. Today terminals of this type, such as the device disclosed in U.S. Pat. No. 4,168,058, which is assigned to the assignee of the present invention, serve in an important capacity in the business and financial communities.
A major function of any currency dispenser is to remove bills from a stack and present them one at a time to a transport mechanism for delivery to the customer. This function may also be deemed the most critical, since problems in delivering the bills, such as failure to feed or the feeding of more than one bill at a time, generally begin at this stage. Because of the wide disparity in bill quality, ranging from new, crisp bills to limp, torn ones, as well as the occurrence of foreign matter between bills, preventing their separation, it is not possible to achieve perfect performance. Therefore, it is important to detect the problem situation and take corrective action immediately, such as the activation of a diverter gate, to capture the erroneously fed bills.
A number of different techniques have been used in the past for detecting the presence of multiple bills or record members in a dispensing system. These systems are mechanical, electrical, or optical devices, or a combination thereof, to sense the presence of multiple bills. Overall performance of a doubles detector, however, depends heavily on the means of determining if a document has been fed, and if so, whether it was a single document.
One method presently employed is based on measuring the optical transmission across the path of the document at a particular time in the machine cycle. A reduction in transmission signals the occurrence of a document in the transport. The amount of reduction will indicate if the document is of the proper thickness and, therefore, a single document. This method involves an optical system, consisting of a light source and sensor, which provides a calibrated output relative to document thickness. However, printed patterns on documents change the amount of light transmission; thus, the system must be calibrated for a particular location on the bill, and timed to read when that position is reached on the bill. Other potential problems which may arise in this type of system are: the light level must be held constant by auxiliary circuitry to provide a reference base; marks or blemishes on the bill can cause reading errors to occur; and the expected transmissivity of a bill changes with its life cycle.
Another method often used in doubles detectors employs a set of pinch rolls, one of which is mounted on a fixed shaft while the other is spring loaded against the first roll. If a bill enters the rolls, the spring mounted roll will move away from the fixed roll by an amount equal to the bill thickness. Measurement of this movement indicates presence of a single bill or multiple thicknesses. Since the thickness of a U.S. bill is approximately 0.010 centimeters, sensitive detectors, such as differential transformers or strain gauges, must be employed to measure the roll position. Also, mechanical linkages common to this method often create hysteresis and vibrational problems. Static changes in the roll position, caused by wear, elasticity, or accumulation of foreign matter, may result in a drift of the system from the operating range. In addition, since the electrical signal is a low level, auxiliary circuits must be used to provide compatible digital signal levels.
Other methods of detecting multiple documents employ short wavelength radiation with relatively complex detection circuitry, or the use of continuous vacuum for separating multiple sheets mechanically. However, all these methods of double document detection often suffer reliability problems in addition to adding significantly to the cost of the dispenser.